This invention relates generally to a waveguide, and a method of manufacturing a waveguide, for use in, for example, communication or radar applications.
Waveguides are generally used for connecting together transmitting and receiving equipment in, for example, communication or radar systems. A waveguide typically comprises a transmission line formed from a hollow conducting tube providing a conduit through which electromagnetic waves are propagated, and may be of any cross-sectional shape, such as, square, rectangular, circular or elliptical, as well as containing single or pairs of opposing ridges.
Regardless of the application in which waveguides are to be used, they are commonly provided with some means of connecting adjacent waveguide sections. The successful connection of waveguides for good Radio Frequency (RF) performance requires that the signals being carried by the waveguides are not significantly reflected, absorbed or distorted by the junction, i.e. the junction offers no appreciable discontinuity to the flow of microwave power. This requires that the internal cross-sections on each side of the junction are well aligned, there is low electrical resistance across the interface and that the point of good contact (i.e. low electrical resistance) occurs at the inside wall of the waveguide, where the electrical currents are generated by the passing signals within.
To this end, various mechanical joints are available, but traditionally, waveguide sections are each provided with flanged ends and connections to components and other discrete waveguide sections are made by using threaded bolts to pull together the flanged ends of the waveguides to form a contacting joint. Flange designs tend to be standardised and, in combination with the above-mentioned fixing bolts, provide the required alignment between waveguides with sufficient mechanical integrity to resist the applied physical forces occurring in service: in a space application, for example, such loads may arise in vibration during launch and differential thermal expansion when in orbit.
In communication systems, where a single waveguide may be carrying high level signals to be transmitted using two or more carrier frequencies, as well as low level received signals, another issue can occur, known as passive intermodulation. Passive intermodulation (PIM) is the generation of interfering signals caused by nonlinearities in the mechanical components of a system, and occurs when two signals mix together (amplitude modulation) to produce sum and difference signals and products within the same band, causing distortion. These effects can occur at waveguide interfaces and a common approach to minimising the effect is to ensure that there is a high contact pressure at the inside wall, usually achieved by providing a raised contact lip around the inside wall so the contact force applied by the fixing bolts is concentrated at this point.
In view of the proven electrical performance of bolted flanges, and similar mechanical joints, together with the reversibility and resultant flexibility thereof, they have long provided the most widely accepted method of joining waveguide sections to each other and/or other interfaces.
However, in modern communication satellites, for example, there can be hundreds or even thousands of waveguide junctions and the resultant mass of flanges and associated fixings can cause significant issues. For instance, the flanges require additional space to be provided at each joint as well as sufficient access to enable the fixing bolts to be tightened. Furthermore, more complex antennas may have many waveguides feeding into them, and in some cases there may be insufficient room for traditional flanged connections.
UK Patent No GB971481 describes a method of joining two waveguide sections together, wherein each waveguide section has, at a connecting end thereof, a respective sleeve section affixed around the waveguide section by means of an adhesive injected into an orifice or ‘pocket’ formed by complementary recesses in the outer wall of the waveguide section and the inner wall of the sleeve section. Subsequently, the two waveguide sections are joined together at sleeve section ends of the waveguide sections by providing a further sleeve member over the butted interface between the sleeve sections, and affixed thereto by means of an adhesive injected into orifices or ‘pockets’ formed by complementary recesses in the outer wall of each sleeve section and the inner wall of the sleeve member.